The significance of 3H-thymidine degradation in cell culture experiments with special reference to rheumatoid arthritis.

The degradation of 3H-thymidine under various cell culture conditions was analysed. It was found that a half of 3H-thymidine was degraded to 3H-thymine during 24 hours in PHA stimulation of blood lymphocytes. A control culture in which PHA was not added also caused 3H-thymidine degradation. 3H-thymidine degradation was prevented by adding 5-nitrouracil to the incubation medium at a concentration of 0.577 mg/ml. At the same time 5-NU increased 3H-thymidine incorporation into lymphocytes by 47%. 5-NU also eliminated the inhibitory effect of rheumatoid arthritis synovial tissue eluate on PHA stimulation. In addition 5-NU and nonradioactive thymine increased the 3H-thymidine labelling index of fresh rheumatoid arthritis synovial membrane biopsies, and also more of the isotope was accumulated in the individual cells of the membrane. These studies demonstrate that 3H-thymidine degradation is an important phenomenon in cell cultures and that it can be prevented effectively by using 5-nitrouracil with 3H-thymidine.